6.1 Collision theory and rates of reaction

  • Chemical Kinetics: The study and discussion of chemical reactions with respect to reaction rates
  • Rate of Reaction: The change in concentration of reactants or products per unit time.

Experimental measurements of reaction rates

  • Δc, the change in concentration can be measured indeirectly by monitoring a property which changes when reactants are converted to products. Examples include:
    • pH (acid – base reactions)
    • Conductivity (reactions with electrolytes)
    • Mass/volume (reactions involving gases)
    • Colour (reactions with transition metals or other colored compounds)
  • To measure reaction rate, plot concentration vs time graph. The rate is determined from slope of gradient at point t on the graph.
  • Rate of reaction can be measured in three ways
    • Average rate
      • Avg rate = Δc/Δt
    • Instantaneous rate
      • Slope of tangent at a point
    • Initial rate
      • Slope of tangent at point t0

Kinetic molecular theory of gases

  • Explains why gases act the way they do
  • Theory
    1. Gases have many particles moving at high velocities in random directions
    2. Size of a gaseous particle is negligible
    3. Collisions between gaseous particles are elastic; no energy is lost
    4. Keav is proportional to the absolute temperature in kelvin

Occam’s razor

  • Used as a guide to develop a theory
  • A principle which states that “Entities should not be multiplies unnecessarily”
    • This means if you have two competing theories, use the simpler one unless there is proof otherwise
  • Collision theory was built using Occam’s razor

Catalysts

  • A substance that increases the rate of a chemical reaction by lowering the activation energy and is not consumed in the reaction.
  • This can be demonstrated using a potential energy profile
  • Potential Energy Profile
  • Activation energy: The minimum energy required for a reaction to occur
  • Catalysts come in two types:
    • Homogeneous:
      • In the same physical state as the reactants
      • Example is destruction of ozone, O3, by chlorine atoms. In the stratosphere, ozone absorbs over 95% of UV radiation from sun. It under goes homolytic fission and converts UV radiation to heat. Chlorine atoms are produced in the reaction of a chlorofluorocarbon (CFC) with UV light.
    • Heterogeneous:
      • In a different state from the reactants
      • Example is catalytic converter in exhaust system of car which converts harmful gaseous into water oxygen or carbon dioxide

Maxwell – Boltzmann energy distribution and temperature

  • Shows the probability of finding particles with a specific kinetic energy
  • Number of particles represented by area of green (in the picture) have enough energy to react
  • For a catalyzed reaction, the activation energy decrease, and there are more particles which have enough energy to react
  • As temperature increases, the mean velocity of particles increases and thus the distribution become flatter and wider
  • Maxwell - Boltzmann

Factors that affect the rate of chemical reaction

  1. Increasing temperature
    • Gives particles more kinetic energy, faster rate of reaction
  2. Addition of catalyst
    • Reduces activation energy, faster rate of reaction
  3. Increasing concentration of reactants
    • Increasing concentration means more collisions, faster rate of reaction
  4. Increasing surface area in solid phase
    • Breaking down solid into smaller pieces means larger overall SA, more collisions, faster rate of reaction

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